1. Field of the Invention
The present invention relates to a gas sensor for use in measurement of the concentration of a hydrogen-containing component gas, and more particularly, to a hydrogen gas sensor suitable for measuring the concentration of a component gas, especially hydrogen gas, contained in a fuel gas for fuel cells.
2. Description of the Related Art
In view of the issue of global-scale environmental deterioration, fuel cells, which are clean and efficient power sources, have recently become the subject of active studies. Among fuel cells, a polymer electrolyte fuel cell (PEFC) is expected to be suitable for vehicle use due to its advantages, including low operation temperature and high output density. In this case, a reformed gas obtained from methanol or the like is advantageously used as a fuel gas. Further, in order to improve efficiency and other performance parameters, a gas sensor capable of directly measuring a hydrogen gas concentration or the like of the reformed gas becomes necessary. Since such a gas sensor is used in measurements performed in a hydrogen-rich atmosphere, the operation temperature of the gas sensor must be low (about 100xc2x0 C. or less).
Such a low-operation-temperature-type hydrogen gas sensor is proposed in Japanese Patent Publication (kokoku) No. 7-31153. In the low-operation-temperature-type hydrogen gas sensor, a working electrode, a counter electrode, and a reference electrode are disposed on an insulating substrate, and the three electrodes are integrally covered with a gas-permeable, proton-conductive film.
Separately, a hydrogen gas sensor which operates at high temperature is proposed in Japanese Patent Application Laid-Open (kokai) No. 8-327592. This hydrogen gas sensor has a structure such that a porous positive electrode layer, a proton-conductive ceramic thin film, and a porous negative electrode layer are successively stacked on a porous ceramic substrate, and operates properly in a state in which the sensor is heated to a high temperature by use of a heater. In this hydrogen gas sensor, the porous ceramic substrate functions as a gas-diffusion-rate limiting layer.
In the hydrogen gas sensor proposed in Japanese Patent Publication No. 7-31153, since a gas under measurement diffuses to the working electrode via the gas-permeable, proton-conductive film which integrally covers the working electrode, the counter electrode, and the reference electrode, diffusion of the gas to the reference electrode cannot be prevented. Even when the reference electrode is formed of a metallic material having a low reactivity with the gas under measurement, the influence of the gas diffusion cannot be eliminated.
In the gas sensor, when the gas permeability of the proton-conductive film is lowered in order to decrease the amount of gas diffused to the reference electrode, a new problem arises in that the amount of gas diffused to the working electrode decreases accordingly, resulting in degraded sensitivity. Further, in the gas sensor, the proton-conductive film must be made porous in order to secure some degree of gas permeability. However, in this case, the mechanical strength of the proton-conductive film is reduced.
The hydrogen gas sensor proposed in Japanese Patent Application Laid-Open No. 8-327592 operates properly only at high temperature, because the proton-conductive layer is ceramic. From the viewpoint of safety, use of this gas sensor in a hydrogen-rich atmosphere is problematic. In addition, in the hydrogen gas sensor, since the porous ceramic substrate functions as a gas-diffusion-rate limiting layer, the strength of the substrate is low. Further, the substrate must be formed to be relatively large, in order to enable formation of the electrodes, the proton-conductive layer, etc. on the substrate through stacking. In a hydrogen gas sensor, such as the gas sensor proposed in Patent Application Laid-Open No. 8-327592, in which the substrate functions as a gas-diffusion-rate limiting layer, the size of the gas-diffusion-rate limiting layer is large, so that breakage of the gas-diffusion-rate limiting layer affects measurement to a greater extent than in the case in which a gas-diffusion-rate limiting layer of small size is used.
It is therefore an object of the present invention to provide a gas sensor which operates at low temperature in a hydrogen-rich atmosphere, and more specifically a hydrogen gas sensor capable of accurately measuring hydrogen gas concentration of a fuel gas of a fuel cell.
A first gas sensor according to the present invention comprises: a proton-conductive layer formed of a polymer electrolyte; first and second electrodes provided in contact with the proton-conductive layer and having a function of dissociating hydrogen gas; a gas-diffusion-rate limiting layer disposed between the first electrode and an atmosphere of a gas under measurement and adapted to diffuse the gas under measurement toward the first electrode in a diffusion-rate limited state; and a support for supporting the proton-conductive layer, the first and second electrode, and the gas-diffusion-rate limiting layer.
When gas concentration is measured by use of the gas sensor, the gas sensor is controlled as follows. By applying a predetermined voltage between the first and second electrodes, a predetermined gas component such as hydrogen gas is dissociated into protons on the first electrode. The thus-produced protons are pumped from the first electrode to the second electrode via the proton-conductive layer. Saturation current which flows from the first electrode to the second electrode as a result of the pumping out is detected, and the concentration of the predetermined gas component is determined on the basis of the saturation current.
In the gas sensor, since the gas-diffusion-rate limiting layer is provided, diffusion of the gas under measurement to the first electrode can be freely controlled with ease, without the necessity of changing the shape of the proton-conductive layer. Therefore, the magnitude of the saturation current, which determines the sensor sensitivity, can be changed freely, so that various measurement ranges can be used selectively.
In the gas sensor, since the diffusion rate of the gas under measurement is limited by the gas-diffusion-rate limiting layer, when the predetermined gas component contained in the gas under measurement has a constant concentration, the magnitude of current flowing between the first and second electrodes becomes substantially constant even when the voltage applied between the first and second electrodes exceeds a predetermined level. When current of a constant magnitude flows irrespective of the magnitude of the applied voltage, the current is called saturation current, and the magnitude of the current is called the xe2x80x9csaturation current value.xe2x80x9d
Further, in the gas sensor, since the sensing portion of the gas sensor is formed on a support formed of an electrically insulative ceramic, the size of the sensing portion can be easily reduced without impairing of the overall mechanical strength of the gas sensor.
A second gas sensor according to the present invention comprises a reference electrode in addition to the structural elements of the above-described first gas sensor. The reference electrode is provided in contact with the proton-conductive layer such that an electrical potential corresponding to a reference hydrogen gas concentration is produced.
When a gas concentration is measured by use of the gas sensor, the gas sensor is controlled as follows. Voltage is applied between the first and second electrodes such that a predetermined voltage is produced between the first electrode and the reference electrode across the proton-conductive layer. Consequently, a gas component to be measured, such as hydrogen gas, which is contained in the gas under measurement and whose diffusion rate is limited by the gas-diffusion-rate limiting layer, is dissociated into protons. The thus-produced protons are pumped from the first electrode to the second electrode via the proton-conductive layer. The concentration of the gas component under measurement is determined on the basis of current which flows as a result of the pumping.
In the gas sensor, the voltage applied between the first and second electrodes can be controlled variably such that a constant voltage is produced between the first electrode and the reference electrode. Therefore, an optimal voltage can be applied to the gas sensor for any concentration of the gas component under measurement, so that accurate measurement can be performed in a wide concentration range.
Further, as in the above-described first gas sensor, in the second gas sensor, diffusion of the gas under measurement is limited by the gas-diffusion-rate limiting layer. Therefore, diffusion of the gas under measurement to the first electrode can be controlled, without changing the shape of the proton-conductive layer. Further, diffusion of the gas under measurement to the reference electrode can be easily prevented. Moreover, in the second gas sensor, freedom of design similar to that in the case of the first gas sensor is possible in relation to the material of the proton-conductive layer and use of the support.
A preferred mode of the present invention will now be described.
In a gas sensor according to the preferred mode of the present invention, a proton-conductive layer is formed of one or more types of fluororesins, more preferably of xe2x80x9cNAFIONxe2x80x9d (trademark, product of Dupont).
In the gas sensor according to a preferred mode of the present invention, the proton-conductive layer is a polymeric electrolytic proton-conductive layer which operates adequately at a relatively low temperature, for example, temperatures not greater than 150xc2x0 C., preferably, at temperatures not greater than 130xc2x0 C., more preferably, at around 80xc2x0 C.; e.g., a proton-conductive layer formed of a fluororesin-based solid polymer electrolyte.
In the gas sensor according to the preferred mode of the present invention, ceramic powder used as a material for the gas-diffusion-rate limiting layer has an average grain size of 2 to 80 xcexcm. Preferably, the gas-diffusion-rate limiting layer is formed of porous alumina.
In the gas sensor according to the preferred mode of the present invention, the first and second electrodes and the reference electrode are formed of a material having a catalytic function. Preferably, these electrodes are formed of a Pt electrode material containing Pt or a Pt alloy as a main component.
In the gas sensor according to the preferred mode of the present invention, the support is comparatively dense, and preferably has a relative density of 95% or more.
In the gas sensor according to the preferred mode of the present invention, the first electrode has a gas-diffusion-rate limiting function. This eliminates the necessity of providing a separate gas-diffusion-rate limiting layer, to thereby further simplify the structure of the gas sensor. In this case, the first electrode is preferably formed of Pt powder having an average grain size of 2 to 50 xcexcm.
The gas sensor according the preferred mode of the present invention can be fabricated as follows. By means of screen printing, a layer of alumina paste, which is to become the gas-diffusion-rate limiting layer, and layers of Pt-containing paste, which are to become the first and second electrodes and the reference electrode, are formed at predetermined positions on an alumina formed sheet, which is to become a support. Subsequently, the formed sheet and the paste layers are integrally fired, and a proton-conductive polymer electrolyte film, which is to become the proton-conductive layer, is bonded to a predetermined position of the fired body by means of hot pressing. Alternatively, a solution of a proton-conductive polymer electrolyte is applied to a predetermined position of the fired body and dried. The first and second electrodes and the reference electrode may be formed by a sputtering method. The method of fabricating the gas sensor according to the present invention is not limited to the above-described method.
In the gas sensor according to the preferred mode of the present invention, the first and second electrodes are formed to sandwich the proton-conductive layer. Alternatively, the first and second electrodes are formed on a common plane. When the first and second electrodes are formed on a common plane, the number of steps for forming the electrodes can be reduced.
In the gas sensor according to the preferred mode of the present invention, the gas-diffusion-rate limiting layer is formed on the surface of the support or within the support. When the gas-diffusion-rate limiting layer is formed (embedded) within the support, the structure of the gas sensor is simplified further.
In the gas sensor according to the preferred mode of the present invention, the reference electrode is formed such that the reference electrode is in contact with the proton-conductive layer, more preferably, is covered by the proton-conductive layer so as not to be exposed to a gas under measurement.